Media filters such as disposable air filters are used to filter intake air in a wide variety of applications. In heating, ventilation, and air-conditioning (HVAC) applications, for example, such media filters are often provided as part of a filtration system for filtering intake air into HVAC equipment such as a fan or blower. A typical residential air circulation system includes a fan, which acts to draw air within the occupied space of the house into an intake opening for reheating or reconditioning. The filter media for such filters may have a pleated configuration that permits particulate contamination to be trapped in the filter without significantly reducing air flow through the filter. In some panel systems, the pleated filter media may be positioned within a rectangular frame or framework system which supports the filter media across the face of the intake opening.
The framework for supporting the filter medium typically includes a number of edges having a U-shaped cross section that encloses the filter medium periphery to form a generally box-shaped structure. In some systems, the media and framework are designed as a unit that can later be disposed after use for a period of time. The dimensions of the filter media and supporting framework will often vary to conform to the dimensions of the intake opening in which the filter is to be installed. In some residential systems, for example, the filter media can comprise a nominally one inch (2.5 cm) thick rectangular mat having length and width dimensions of between one to two feet (30.5 cm to 61 cm).
The design of many media filters typically includes considerations such as air flow resistance and efficiency. Lower air flow resistance, measured as the pressure drop across the filter, is typically desirable since less energy is required to move a given amount of air through the filter in comparison to a filter with high air flow resistance. Increased efficiency is also desirable to remove more particulates from the air stream. In general, as the filtration efficiency of the filter increases, the pressure drop across the filter likewise increases. Since the pressure drop is proportional to the air flow rate through the filter media, one method employed to lower pressure drop without decreasing efficiency is to use a pleated filter medium. While the use of a pleated filter medium increases the surface area to better trap particulates contained in the air stream, such designs often increase the depth of the filter, in some cases to a depth of four inches (10.2 cm) or more.
The shipping and storage costs associated with many prior art media filters, including many pleated designs, often represents a significant expense in the total cost of the media filter. For pleated filters, for example, the existence of air between the pleat folds can increase the bulk filter volume required to store such filters in warehouses, on retail or distributor shelves, and/or on a contractor's truck, thus increasing distribution costs. In addition, such increase in bulk filter volume can increase the costs associated with packaging the filter.
To reduce filter volume, collapsible filters have been introduced to permit the pleated filter media to be reduced in size during periods of nonuse. In some designs, for example, the collapsible filter may include a number of detachable frame members that permit the pleated filter medium to be collapsed in an accordion-like fashion, allowing the filter medium to occupy substantially less volume than a fully expanded filter. Although such collapsible filters can be used to reduce bulk filter volume when not in use, the assembly of such devices by the installer is often difficult and time consuming. In addition, in those instances where the frame members are intended to be reusable, the removal of such frames from a dirty filter can be unpleasant. For HVAC contractors, the time required to assemble the filter can add to the total cost of installation.